The most important news from 2015 in fundamental physics is that probably there is no news. Let me explain. With one tantalizing exception (which may be a statistical anomaly), the experiments done recently confirm a frustratingly incomplete theory of fundamental physics, which has stood since the 1970s. This is in spite of enormous effort by thousands of experimentalists hoping to discover new phenomena that would lead to greater unification and simplification in our understanding of nature.
Since 1973, our knowledge of elementary particles and fundamental forces has been expressed in what we call the Standard Model of Elementary Particle Physics. This reduces all phenomena, save gravity, to twelve fundamental particles interacting via three forces. This Standard Model has been confirmed in all experiments to date. This includes new measurements announced this month by two teams of experimentalists operating the Atlas and CMS detectors at the Large Hadron Collider (LHC), working at nearly twice the energy as previous experiments.
In 2012, the news from the LHC was the discovery of the Higgs, which was the last particle predicted by the Standard Model remaining to be discovered. But the Standard Model cannot be the whole story, in part because the model involves twenty-nine free parameters. We have no explanation for the values of these parameters and hence seek a deeper theory “beyond the Standard Model” that would explain them. Moreover, many of these values seem extremely unnatural: they are very tiny numbers with large ratios among them, (the hierarchy problem) and they seem to be tuned to special values needed for a universe with many stable nuclei allowing complex life to exist (the fine tuning problem). In addition, there is no reason for the choices of the fundamental particles and for the symmetries that govern the forces between them. Another reason for expecting new particles beyond the Standard Model is that we have excellent evidence from astronomy for dark matter, which gravitates but doesn’t give off light. All these pieces of evidence point to new phenomena that could have been discovered at the LHC.
Several beautiful hypotheses have been offered on which to base a deeper unification going beyond the Standard Model. I will just give the names here: supersymmetry, technicolor, large extra dimensions, compositeness. These each imply that new particles should have been discovered at the LHC. Some also point to more exotic phenomena such as quantum black holes. To date, the experimental evidence sets impressive limits against these possibilities.
To be sure, there is one weak, but very exciting, indication from new results that might be interpreted as a signal of a new particle beyond the Standard Model. This is a small excess of collisions which produce pairs of photons that, remarkably, are seen by both of the experiments operating at the LHC. But the statistical significance is not high when the statistics is done taking into account that one is bound to get some signal by random chance in one of the many channels looked at. So it could be a random fluctuation that will go away when more data is taken.
Even if this hint grows into the discovery of a new particle, which would be extremely exciting news, it is too soon to say whether it will lead to a deeper unification, rather than just add complication to the already complicated Standard Model. Luckily, more data can be expected soon.
It is the same in quantum gravity, which is the unification of quantum theory with Einstein’s theory of gravity. Many proposals for quantum gravity suggest that at certain extremely high energy scales we must see new physics. This would indicate that at correspondingly tiny scales space becomes discrete, or new features of quantum geometry kick in. One consequence would be that the speed of light is no longer universal—as it is in relativity theory—but would gain a dependence on energy and polarization visible at certain scales.
In the last decade this prediction has been tested by sensitive measurements of gamma rays that have traveled for billions of years from extremely energetic events called gamma ray bursts. If the speed of light depends even very slightly on energy, we would see higher energy photons arriving systematically earlier or later than lower energy photons. The enormous travel time would amplify the effect. This has been looked for by the Fermi satellite and other detectors of gamma rays and cosmic rays. No deviations from relativity theory are seen. Thus, our best hope of discovering quantum gravity physics has been frustrated.
A similar story seems to characterize cosmology. Something remarkable happened in the very early universe to produce a world vast in scale but, at the same time, extremely smooth and homogeneous. One explanation for this is inflation—a sudden enormous expansion at very early times, but there are competitors. Each of these theories requires delicate fine tuning of parameters and initial conditions. Once this tuning is done, each predicts a distribution of noisy fluctuations around the smooth universe. These show up as a seemingly random distribution of very slightly denser and less dense regions. Over hundreds of millions of years of expansion, these amplify and give rise to the galaxies. These fluctuations make bumps that are visible in the cosmic microwave radiation. So far, their distribution is as random, featureless, and boring as possible, and the simplest theories—whether inflation or its alternatives—suffice to explain them.
In each of these domains we have sought clues from experiments into how nature goes beyond, and solves the puzzles latent in, our incomplete theories of the universe, but we have so far come up with nearly nothing.
It is beginning to seem as if nature is just unnaturally fine tuned. In my opinion we should now be seeking explanations for why this might be. Perhaps the laws of nature are not static, but have evolved through some dynamical mechanism to have the unlikely forms they are observed to have.